Very often the motivation for *buying* monoblocks is that there is no choice if one wants a certain amplifier or manufacturer. There are many great stereo amps that cannot be had as monos and vice-versa.
In very high-power and/or class-A designs, practicality in terms of size & weight is indeed the driver. Imagine a 600W Pass stereo amp, or a parallel-845 SE stereo amp (wait - there really is one of those).
Some of the best amplifiers I've had in terms of stage, imaging, and channel separation have indeed been stereo amps, and not always dual-mono either! (True dual mono, with two power trannies, is extremely rare.)
For people that swap gear a lot (not me - but I know some of them) - going back and forth between stereo & mono amps can be a real pain because of the different speaker cables and rack requirements.
The smartest approach if absolute performance with yet with some salute to economy is desired may be a stereo amp with the HV supply in its own chassis - think ASR.
